1. Field of the Invention
The present invention generally relates to acousto-optical devices, and more particularly to an acousto-optical device having a light waveguide path formed on an acousto-optical substrate and a transducer which crosses the acousto-optical waveguide path and propagates a surface acoustic device along the acousto-optical waveguide path, in which various mutual actions are caused in a light wave propagated through the light waveguide path under a surface acoustic wave controllable by an electric signal applied to the transducer.
An optical filter device is used in terminal equipment or a repeater or relay device in an optical communication system in order to separate signal lights which are transmitted in a wavelength-multiplexed formation. The mutual action of a surface-acoustic wave and light can realize a tunable wavelength filter, and the optical system can flexibly be constructed.
Examples of typical filter structures utilizing the surface-acoustic wave are as follows. A structure uses a TE-TM mode transducer which transduces a TE/TM wave of a wavelength input light to a TM/TE wave in combination with a polarizer which extracts a particular polarized wave. Another structure uses the Bragg diffraction. Yet another structure uses a coupling of the even and odd modes in a directional coupler. The above structures can realize a light-intensity modulator and an optical switch in such a way that the structures do not function as a filter.
2. Description of the Related Art
FIG. 1A is a perspective view of a conventional TE-TM mode transducer. As shown in FIG. 1A, the transducer is made up of an acousto-optical substrate 1, high-density Ti diffused areas 2a and 2b, a diffused light waveguide path (channel) 3, an interdigital transducer 4, and acoustic-wave absorbers 5a and 5b. The acousto-optical substrate 1 is made of, for example, an X-cut plate (Y-axis propagation) of LiNbO3. The transducer 4 excites a surface acoustic wave (SAW) in an area including the waveguide path 3, and has finger electrodes 4a and 4b formed of a metal such as aluminum. The absorbers 5a and 5b are made of an acoustically soft material such as wax or rubber.
The high-density Ti diffused areas 2a and 2b are located on both sides of the substrate 1 and function to increase the acoustic velocity therein. Hence, SAW power is contained within the surface area of the substrate 1 sandwiched between the areas 2a and 2b. 
The Ti waveguide path 3 provided in the longitudinal direction of the substrate 1 and located in the center thereof is formed by thermally diffusing Ti. The thermal diffusion method can change the refractive indexes no and ne of the LiNbO3 substrate with respect to ordinary light (ray) and extraordinary light by an almost identical degree.
The SAW is generated by utilizing the piezoelectricity of LiNbO3 in such a way that an RF (high frequency) signal is applied across the finger electrodes 4a and 4b of the transducer 4 which is directly mounted on an end surface portion (light input side) of the substrate 1. The distance 1 between the finger electrodes 4a and 4b and the wavelength xcex9 of the SAW has a relationship such that 1=xcex9/2. In this case, the SAW power generated in the substrate 1 is defined by multiplying the RF signal input power by an efficiency. The SAW oriented to the light input side is acoustically absorbed by the absorber 5a and thus disappears immediately. The SAW directed to the light output side is propagated on the substrate portion between the areas 2a and 2b at an acoustic velocity v.
In a case where a polarized wave of TE-mode (or TM-mode) light is applied to the input end of the waveguide path 3 in the above state, the plane of polarization of the polarized wave is turned by 90xc2x0 due to the acoustic-optical effect of the SAW propagated on the substrate 1 when the wave has traveled a given action length L. Hence, the polarized wave of TE-mode (or TM-mode) light is transduced into that of TM-mode (or TE-mode) light. The above rotation can be controlled by the power of the SAW. The absorber 5b is located in the above position. Hence, the mutual action to the SAW does not occur in the waveguide path 3, and thus the polarized wave of the TM (or TE) mode can be obtained via the output end of the waveguide path 3.
The following phase matching condition is satisfied in the above case:                                                                         "LeftBracketingBar"                                                      β                    TE                                    -                                      β                    TM                                                  "RightBracketingBar"                            =                              xe2x80x83                            ⁢                                                (                                      2                    ⁢                                          π                      /                      λ                                                        )                                ⁢                                  "LeftBracketingBar"                                                            N                      TE                                        -                                          N                      TM                                                        "RightBracketingBar"                                                                                                        =                              xe2x80x83                            ⁢                                                2                  ⁢                                      π                    /                    Λ                                                  =                                  2                  ⁢                  π                  ⁢                                      xe2x80x83                                    ⁢                                      f                    /                    v                                                                                                          (        1        )            
where xcex2TE and xcex2TM respectively denote the propagation constants of the waveguide modes TE and TM, NTE and NTM respectively denote the effective refractive indexes of the modes TE and TM, xcex9 denotes the wavelength of the SAW, f denotes the frequency, and v denotes the phase velocity. The mode transduction is caused due to the SAW of the frequency f which satisfies equation (1), and the transduction efficiency can be controlled by the SAW power.
The following equation (2) can be obtained from equation (1):
xe2x80x83xcex=xcex9|NTExe2x88x92NTM|xe2x80x83xe2x80x83(2)
A numerical example will be described below. The index of double refraction |NTExe2x88x92NTM| obtained used when LiNbO3 is approximately equal to 0.072. In order to realize the above mode transduction with light having a wavelength xcex of 1.55 nm (frequently used in optical communications), the wavelength xcex9 of the SAW is equal to 21.5 xcexcm. Since the acoustic velocity (phase velocity) v on the substrate 1 is approximately equal to 3700 m/s, the RF signal is required to have a frequency f (=v/xcex9) of 172 MHz. The power of the RF input signal depends on the mutual action length L to the SAW. Assuming that L=30 mm, the RF power is approximately equal to 10 mW.
With the above arrangement, the TE/TM wave of the input light can efficiently be transduced into the TM/TE wave by a reduced RF signal level and reduced RF power.
A wideband acousto-optical tunable wave filter can be configured by providing, at the following state, a polarizer for extracting the TM (or TE) wave.
In the structure shown in FIG. 1A, the finger electrodes 4a and 4b of the transducer 4 are directly mounted on the surface of the waveguide path 3. Hence, the light propagated through the waveguide path 3 is absorbed by the finger electrodes 4a and 4b due to the presence of the metal forming them, and thus the light has a considerable propagation loss. This is because metal has a negative dielectric constant and serves as a dielectric having a large loss due to the inertia effect of charges in metal in the light wavelength range. Particularly, the electromagnetic field distribution in the TM mode enters deeply in metal, and is greatly affected by metal (the degree of influence in the TM mode is approximately ten times that in the TE mode).
In order to reduce the propagation loss of light caused by the transducer 4, an improved arrangement has been proposed as shown in FIG. 1B. A buffer layer 6 is provided between the entire area between the transducer 4 and the substrate 1 and is formed of a dielectric film such as SiO2. The buffer layer 6 reduces the influence resulting from the presence of metal (transducer 4). As the thickness of the buffer layer 6 is increased, the propagation loss of the light propagated through the waveguide path 3 is drastically reduced. In a case where the TMo mode light can be propagated through a single-mode waveguide path, if the SiO2 film is 0.16 xcexcm or more in thickness, the propagation loss can be reduced to 0.1 dB or less.
However, the structure shown in FIG. 1B has a disadvantage in that the presence of the buffer layer 6 provided continuously between the transducer 4 and the substrate 1 greatly reduces the efficiency in excitation of the SAW, and an increased RF power is needed. This is because a sufficient intensity of electric field cannot be applied to the substrate 1 due to the electrically insulating performance of the buffer layer 6 and mechanical stress of the buffer layer 6 functions to prevent occurrence of the SAW and propagation thereof.
It is a general object of the present invention to provide an acousto-optical device in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide an acousto-optical device having a reduced light absorption loss and an increased efficiency in exciting the SAW.
The above objects of the present invention are achieved by an acousto-optical device comprising: a light waveguide path formed on an acousto-optical substrate; a transducer which crosses the light waveguide path and propagates a surface acoustic wave along the light waveguide path; and a buffer layer provided so that finger electrodes of the transducer are spaced apart from the light waveguide path in crossing portions in which the finger electrodes cross the light waveguide path. The finger electrodes have other portions which directly contact the substrate. The acousto-optical structure of the above transducer can generate various mutual actions in light propagated through the light waveguide path. Examples of these mutual actions are colinear coupling with two waves propagated in parallel (the same direction coupling, reverse direction coupling), a mode transduction in which an output wave having a mode different from that of an input wave, TE-TE mode coupling, TM-TM mode transduction, TE-TM mode coupling, and TM-TE mode transduction.
According to the above acousto-optical structure, the portions of the finger electrodes which cross the light waveguide path are spaced apart from the light waveguide path via the buffer layer. Hence, a light absorption loss caused by an influence of a metallic material forming the finger electrodes can be greatly suppressed. The remaining portions of the finger electrodes directly contact the acousto-optical substrate. Hence, the input power of the transducer can efficiently be transduced into SAW power on the acousto-optical substrate. Hence, it is possible to realize an acousto-optical device of low loss and high driving efficiency and thus provide various mode transducers and tunable optical wave filters.
The above acoustic-optical device may be configured so that the buffer layer has a band shape which is continuously provided to the crossing portions. The buffer layer having a band shape is simple and is thus produced easily. It is not required to arrange the buffer layer and the finger electrodes with a high accuracy. Further, it is enough for the buffer layer to have a width slightly greater than the width of the light waveguide path. Hence, the device has a small mechanical stress to generation and propagation of SAW power on the surface of the substrate.
The above acousto-optical device may be configured so that the buffer layer has portions separately provided to areas respectively including the respective crossing portions. Hence, a further reduced mechanical stress to the generation and propagation of the SAW power can be obtained. This contributes to reducing the mechanical fatigue of the portions of the buffer layer.
The acousto-optical device may be configured so that: the buffer layer has a transparency to an input light applied to the acousto-optical device; and the buffer layer has a refractive index smaller than that of the acousto-optical substrate. Hence, the buffer layer functions as a high quality clad layer with respect to input light. The transparency of the buffer layer does not attenuate exudation light from the light waveguide path.
The acousto-optical device may be configured so that the finger electrodes are provided so that the buffer layer is sandwiched between the finger electrodes, and contact the acousto-optical substrate. Hence, the surface acoustic wave can efficiently be excited on both sides of the substrate between which the buffer layer is provided. Further, the surface acoustic wave becomes a single plane wave (or a wave spread in an arc formation) due to the diffraction effect, which is propagated through the acousto-optical substrate.
The acousto-optical device may be configured so that a length of first portions in which the finger electrodes contact the substrate is equal to or greater than twice another length of second portions in which the finger electrodes contact the buffer layer substrate.
The acousto-optical device may be configured so that the buffer layer is formed of a space.